When it comes to tattooist apprenticeships and your portfolio, always take any advice even when harsh as constructive criticism. As for the "darker shades in your art", that is your preference because it is your art. Basically, when it comes down to a client asking you for a sketch, it is up to the client whether or not your shades are dark enough.
Trust me, they will let you know. It is important to be confident, but not to the point where you portray arrogance. Nobody wants to work with a prick.
As for your portfolio, only put your best work in it. The work you leave out is the work that you second guess yourself on. If you don't have confidence in your work, others won't either.
If these guys are offering you pointers, then don't waste anytime grabbing your sketch pad and paying them a visit. This will give them the vibe that you are serious and interested in learning. Be sure you meet up with the head tattooist.
Last, but nowhere near least, it wouldn't hurt to get your bloodborne pathogen training out of the way. You can take the course online or, most times, at your local health department. Fifty dollars is a small price to pay to save a life.
I think that you need to go back and talk to the head tattoo guy, and let them get to know you and to show that you are interested, and confident in your work. Take the advise that you received and see if you can rework your drawings to be more 'tattoo worthy', and draw some new stuff to take in also. Be persistent, but not buggy, and smile and be friendly and listen well and show that you want to learn and will follow their advise etc. luck.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.